Recently, optical discs have been used in many fields as information recording media. The advances in laser oscillator technology have contributed to optical discs having superior high density and high speed characteristics. Information is recorded on these discs in the form of a series of pits and the change in reflectivity of light irradiated onto the disc is used in order to read the recorded information. However, when executing the recording and reading of information on the disc, a focus servo is required for the control of the irradiated light so that it can be accurately focussed on the optical disc. In the past, various methods have been devised for detecting focus in order to operate this focus servo.
For instance, as disclosed in the Japanese Laid-Open Patent Application 59-90237, there is shown a device which performs the detection of focussing errors by detecting the light from an optical disc via a knife wedge masking plate provided in the optical path which provides extraction asymmetrically about the optical axis using a photo detector. In this detection method, the focus servo operates by detecting the focal point using the fluctuation of the position of the light reflected from the optical disc to the photo detector due to the distance between the optical disc and the objective lens. In this device, the photo detector is located at the image formation point of the light reflected from the optical disc. This photo detector has two photo detecting elements.
For another instance, in the Japanese Laid-Open Patent Application 51-141651, there is shown a device in which light from an optical disc is irradiated onto a photo detector, which is divided into four parts, with these parts combined in a square, via a cylindrical lens which generates astigmatism. This photo detector performs detection of focussing errors and generates focus error signals and reading information according to the area of light irradiated. In this detection method, the photo detector is located at the position of the minimum light spot formed between two focal lines which are formed in the direction of the generatrix generated by the cylindrical lens and the direction orthogonal to that generatrix. Detection of focussing errors signals and reading of information are carried out by the same photo detector.
However, in detecting the focal point using the detection method according to the Japanese Laid-Open Patent Application 59-90237, the size of the beam spot of light on the light detecting apparatus becomes small because the photo detector is positioned at the image formation point of the light from the optical disc. For this reason, it is liable to be affected by light axis divergency and the like due to deformation of the optical elements through temperature changes and the like. Thus, there is the disadvantage that accurate focussing control cannot be executed.
In the case of detection using the detection method according to the Japanese Laid-Open Patent Application 51-141651, although focussing error detection and reading information can be executed by the same photo detector, since this photo detector is arranged at the position of the minimum light spot formed between two focal lines generated by the cylindrical lens, accurate detection cannot be performed due to the influence of diffraction of the light.